1. Field of the Invention
The present invention relates to an ink jet recording apparatus and an ink jet head that eject a plurality of colors of liquid droplets.
2. Description of the Related Art
A color ink jet head disclosed in Japanese Patent Laid-Open No. 2001-171119 is mounted on a reciprocating carriage and can perform printing during both the forward and backward strokes of the carriage (bidirectional configuration).
The example of a bidirectional ink jet head disclosed in Japanese Patent Laid-Open No. 2001-171119 has ink tanks that contain three colors of ink: cyan (hereinafter referred to as C), magenta (hereinafter referred to as M), and yellow (hereinafter referred to as Y). To the three ink tanks are connected five liquid chambers, which are arranged side by side in the back surface of a recording element substrate that ejects liquid droplets.
For example, as shown in FIG. 8, liquid chambers 3a, 3b, 3c, 3d, and 3e are filled with C, M, Y, M, and C ink, respectively. Y ink is supplied to the Y liquid chamber 3c from a Y ink tank 21b through a non-branching flow passage 6. C ink is supplied to the two separate C liquid chambers 3a and 3e from a C ink tank 21c through a flow passage divided into two branches 4a and 4e. Similarly, M ink is supplied to the liquid chambers 3b and 3d from an M ink tank 21a through a flow passage divided into two branches 5b and 5d. 
For descriptive purposes, a liquid chamber corresponding to a non-branching flow passage that connects an ink tank to a liquid chamber will hereinafter be referred to as “non-branching liquid chamber.” Liquid chambers corresponding to a branching flow passage that connects an ink tank to a plurality of liquid chambers will hereinafter be referred to as “branching liquid chambers.”
In an ink jet head, suction recovery is performed by a purge mechanism in order to discharge bubbles staying in liquid chambers to which liquid is supplied, ejection ports that eject liquid, and flow passages that connect the ejection ports and the liquid chambers, and to fill them with fresh ink.
In particular, bubbles staying in the flow passages and the liquid chambers narrow the ink passages and decrease the performance of ink supply to the ejection ports, and therefore the bubbles are to be reliably removed.
FIG. 8 shows an example of a relatively simple purge mechanism suitable for an ink jet head that includes ejection port arrays corresponding to branching liquid chambers and an ejection port array corresponding to a non-branching liquid chamber, the ejection port arrays being arranged close to each other, as in a bidirectional configuration. By a simultaneous suction operation using a single suction cap 22, the same negative pressure is exerted on the non-branching liquid chamber and the branching liquid chambers, and ink and bubbles are simultaneously discharged through each ejection port array.
With reference to FIGS. 9A to 9D, how a bubble is sucked out of a liquid chamber during a suction operation, will be described.
Reference numeral 9 denotes the entrance of a liquid chamber 3 at which a flow passage is connected to the liquid chamber 3. Before the suction by the purge mechanism, a bubble 10 stays at the entrance 9 (FIG. 9A). The bubble 10 is formed by the union of small bubbles that come from the upstream side (the ink tank side) of the flow passage with consumption of ink during ejection of liquid droplets, and small bubbles formed near an ejection port array 8 (an arrangement of a plurality of ejection ports 8a in a line).
One of the causes of the stagnation of the bubble 10 at the entrance 9 of the liquid chamber is that the ink flow speed in this part steeply changes due to the change in cross sectional area of the flow passage. The stagnation or behavior of a bubble can be visualized using a CT scanner (FIG. 9B).
When a suction operation using a suction cup is performed from the undersurface 7 of the recording element substrate 2, the bubble 10 is deformed by the flow of ink from the ink tank to the ejection port array 8 (FIG. 9C). When part of the bubble 10 comes into contact with the ejection port array 8, the meniscus of the bubble 10 breaks, and the bubble 10 is discharged together with ink.
In the case of the non-branching liquid chamber, all of the ink flowing from the ink tank flows into a liquid chamber during a suction operation, and the bubble at the entrance 9 of the liquid chamber can be discharged relatively easily.
However, in the case of a system in which ink is supplied from the same ink tank to two liquid chambers through a branching flow passage, a phenomenon is often observed in which only a bubble in one of the two liquid chambers is discharged and a bubble in the other liquid chamber stays at the entrance 9 of the liquid chamber and is not discharged (FIG. 9D).
This is attributed to the fact that when a bubble in one of the two liquid chambers is discharged during a suction operation, the flow resistance of the liquid chamber decreases, and the ink flow in the other liquid chamber in which a bubble stays, decreases. Therefore, even if the suction operation is continued, the ink in the liquid chamber from which a bubble has been removed is mainly discharged, and ink is hardly discharged out of the liquid chamber in which a bubble stays, and therefore the bubble is not removed.
Bubbles in the system of a branching flow passage and a bubble in the system of a non-branching flow passage can be reliably discharged if each system is individually sucked. However, this method complicates the purge mechanism. Particularly in the case of an ink jet head in which ejection port arrays are arranged close to each other as in the bidirectional configuration, a small and precise suction cap that individually sucks each ejection port array is needed. However, such a suction cap is difficult to make.
To discharge bubbles in both liquid chambers connected to a branching flow passage by sucking a plurality of ejection ports at the same time, a more powerful suction operation needs to be performed, for example, at a larger suction pressure (negative pressure). However, in that case, a needlessly large amount of ink is sucked out of the non-branching liquid chamber, which is sucked at the same time as the branching liquid chambers, and the amount of waste ink that is not used for printing increases.